The following problems will be investigated: (1) Is the heteorogeneity of the light chains of antibody molecules and of other immunoglobulins caused by differences in the structure of these immunoglobulins produced in different organs? or is the heterogeneity brought about by secondary changes such as the action of enzymes in vivo or in vitro?---(2) If a protein-carrier coupled with different haptens is injected, two different, haptenspecific antibodies are formed; which of the amino acids are particularly involved in the differences between the two antibodies?---(3) We discovered recently (S. E. Zimmerman and F. Haurowitz, Immunochemistry), that injection of As-TMA-BSA (equals bovine serum albumin substituted by p-azophenylarsonate and p-azophenyl- N-trimethylammonium residues) into heterozygous rabbits of the a1/2, a1/3 or b4/5 allotypes results in the formaton of anti-TMA and anti-BSA antibodies in the four allotypes a1, a2, b4 and b5; however, only anti- TMA and anti-BSA are distinctly expressed in the allotype a3. Anti-As antibodies in an a1/3 heterozygote are expressed preferentially in the a1 allotype, and very weakly in the a3 allotype. The structural bases for this phenomenon of 'preferential expression' in one of the two allotypes in heterozygous animals is not yet clear. It may be responsible for different responsivity of a heterozygous human or animal individual to certain antigens. We intend to investigate the structural bases of this phenomenon of preferential expression.